Pulsed time-of-flight radar gauges are utilized for the measurement of level in process storage tanks. These gauges are mounted on the top of storage tanks, and transmit a pulse of microwave energy toward the surface of the product being stored in the tank. The gauge then receives energy which is reflected from the surface of the product, and calculates the level of the product based upon the time of flight of the pulse.
A PRIOR ART circuit 110 for creating the transmit and sample clocks is shown in FIG. 2. Circuits of this type are described in U.S. Pat. No. 5,563,605 McEwan. This circuit has the advantage that only one oscillator 112 is required, as the sample clock 114 is generated by continuously increasing phase delay in a variable delay generator 116 controlled by a delay control ramp 118. The phase delay circuit can be designed to be repeatable, therefore, errors due to changes in the difference frequency are reduced using this circuit design. However, this circuit has the disadvantage of having significant phase jitter or instability in the sample clock. This is a result of performance limitations of the high speed comparator required as part of the phase delay generator.
There is a need for a radar gauge circuit that is stabilized without the use of expensive, complex circuitry.